This invention relates to an energy saving circuit for controlling the voltage applied to an induction motor as a function of both the motor load and the power source or line voltage.
When an induction motor is operating at a given output or load level, the input voltage to the motor typically varies with fluctuations in the power supply or line voltage. Generally speaking, a reduction of the voltage applied to the motor when the load is small and an increase of the applied voltage when the load is large tend to reduce the overall power consumed by the motor. The regulation of the voltage applied to the motor winding in accordance with the magnitude of the load driven by the motor can thus implement considerable power savings.
A prior art power factor control apparatus for an alternating current induction motor is disclosed in U.S. Pat. No. 4,052,648 to Nola, and includes the following features:
(a) current sampling means connected with each phase winding of the motor for providing an AC output signal in phase with the current through the winding, PA1 (b) voltage sampling means for sensing the voltage of an electrical input applied to the winding and for providing an output signal in phase with the voltage across the winding, PA1 (c) phase detection means responsive to the outputs of the current and voltage sampling means for providing an output which varies in accordance with the phase difference between the sampled current and voltage signals, and PA1 (d) control means connected in series with each motor winding and responsive to the output of the phase detection means for varying the duration of the "on" time of each input power cycle to the motor winding in inverse proportion to the difference in phase between the sampled current and voltage whereby an increase in the difference between the magnitude of the voltage and the magnitude of the load applied to the motor is compensated for by a reduction in power to the motor, generally improving its efficiency.
The power factor control apparatus of U.S. Pat. No. 4,052,648 thus provides, as set forth above, an inversely proportional relationship of the phase difference between the current flowing through each phase winding of an AC induction motor and the input voltage applied thereacross for controlling the conduction period in each cycle. Stated another way, the phase difference determination controls the conduction period in an inversely proportional manner.
The phase difference between the current flowing to a winding of an induction motor and the input voltage applied thereacross is affected by both the magnitude of the input voltage and the load on the motor. It is thus possible that although the input voltage magnitude and the motor load both differ or change, the phase difference remains the same. As a result, this prior art method wherein the phase difference determines the conduction period in an inversely proportional manner does not always maximize the efficiency of the motor operation.